By Michael Peace, CEng MIET MCIBSE, Senior Engineer, Technical Regulations, Institution of Engineering and Technology
Why did the requirements for RCD testing change in BS 7671:2018+A2:2022?
Different types of RCDs are available providing varying levels of DC immunity. Regulation 531.3.3 of BS 7671:2018+A2:2022 only permits the use of Type AC RCDs to serve fixed equipment, where it is known that the load current contains no DC components. Effectively this means resistive loads only, this is because DC components can saturate the coil and ‘blind’ RCDs and reduce their sensitivity. For further information on RCD types, see IET Wiring Matters article, ‘Which RCD Type?’.
The testing requirements for RCDs provided in BS 7671 previously are for Type AC RCDs and taken from the relevant product standards, BS EN 61008 and BS 61009. If all of the tests in the product standards for the different types of RCD were applied to BS 7671 it would have had to become much more complicated and the effectiveness of this level of testing was questionable. A discussion took place to consider the benefits of RCD testing and it was agreed that a single test would be sufficient to verify the effectiveness of automatic disconnection of supply by RCDs.
What are the changes to RCD testing in BS 7671:2018+A2:2022?
The time/current performance criteria for RCDs to BS EN 61008 and BS EN 61009 previously located in Appendix 3 of BS 7671 has been deleted. The information relating to RCD testing is now located in a note of Regulation 643.7 for protection by automatic disconnection of the supply and Regulation 643.8 for additional protection, as shown below.
NOTE: Regardless of RCD Type, effectiveness is deemed to have been verified where an RCD disconnects within the time stated below with an alternating current test at rated residual operating current (IΔn):
• For general non-delay type, 300 ms maximum
• For delay ‘S’ type RCD, between 130 ms minimum and 500 ms maximum.
The important parts to remember are highlighted in bold above. “Regardless of RCD Type” means that it does not matter if a Type AC, Type A, Type B or Type F RCD is to be tested, an alternating current test at rated residual current (IΔn) is carried out and a maximum operating time of 300 ms is expected.
What is an alternating current test?
An alternating current test refers to the type of current used for the test. All types of RCDs have the characteristic of an AC type RCD, this is the reason for selecting an alternating current test.
Modern instruments have the facility to test different types of RCD, the setting is indicated with an image of the RCD type symbol on the test instrument’s screen as shown below. It would be an easy mistake to make to select the Type A setting on the instrument to test a Type A RCD. If the Type A setting is selected, a multiplier of 1.4 is applied to the test current value. So, if the Type A setting is selected to test an RCD with a rated residual current of 30 mA, a test current of 42 mA would be applied. It’s, therefore, important to ensure the correct setting is selected or you could obtain erroneous values.
What are the other RCD testing settings on the instrument used for?
Whilst it is not a requirement of BS 7671:2018+A2:2022, other types of RCD testing may be useful for fault finding purposes as shown below.
• ½ x IΔn
• 5 x IΔn
• Ramp test
• Phase angle (0° & 180°)
• Specific tests relevant for RCD type
For further information on the changes to RCD testing, see IET Wiring Matters article Changes to RCD testing in BS 7671:2018+A2:2022.
Where should you be using RCDs? When should you be using them and why do the regulations keep on changing? Darren and Dave get technical with Michael Peace of the IET to answer some of your questions about residual current devices.